The present invention relates to a polarization beam splitter and a projection apparatus having the same. The present invention relates to a polarization beam splitter with wavelength selectivity which transmits s-polarized light in a first wavelength band and reflects the s-polarized light in a second wavelength band different from the first wavelength band, and reflects p-polarized light in the first wavelength band and transmits the p-polarized light in the second wavelength band, by way of example. The present invention is particularly preferable for a projection apparatus which employs a light modulator with liquid crystal.
A conventionally well-known polarization beam splitter is a polarization beam splitter (PBS) of a prism type in which polarization splitting is achieved by a dielectric thin film comprised of a number of alternately laminated H layers and L layers between two prisms. Each H layer is formed of a dielectric thin film with high refractive index and each L layer is formed of a dielectric thin film with low refractive index.
The dielectric thin film has the optical characteristic of transmitting p-polarized light and reflecting s-polarized light incident thereon. The principles of the polarization beam splitter are that, for p-polarized light, the incident angle generally matches the Brewster angle θB expressed by the relationship of the refractive index nP of the material of the prism, the refractive index nH of the H layer, and the refractive index nL of the L layer, thereby transmitting the p-polarized light. For s-polarized light, the reflection at the interface between the H layer and the L layer is used to reflect the s-polarized light through multilayer film interaction.
The characteristic of the PBS is degraded when it is used out of design conditions due to variations in factors because of a change in incident angle or wavelength for use. In particular, the conditions for the Brewster angle are sensitive to each constant, so that the p-polarized light is more likely to be affected than the s-polarized light.
In an optical system for use in an image projection apparatus (a projection apparatus), luminous flux radiated from a light source often has a certain angular range, and a wavelength range as wide as the whole range of visual light is used. In general, the number of the layers in the polarization splitting film is added or the thickness of the film is modified to provide favorable characteristics in an intended angular range and wavelength band.
A PBS which reflects p-polarized light and transmits s-polarized light has been reported in “Li Li and J. A. Dobrowolski, Appl. Opt., vol. 39, p. 2754, 2000” (hereinafter referred to as “Document 1”). The incident angle is set to an angle equal to or larger than the critical angle of a prism with high refractive index (a prism made of a material with high refractive index) and a thin film with low refractive index (a thin film made of a material with low refractive index) to produce attenuated total reflection. Since the light after attenuated total reflection has its phase changed, the principles of transmitting p-polarized light and reflecting s-polarized light through interference in a provided multilayer film are used to realize the PBS. This achieves favorable characteristics in a wide range of incident angles.
On the other hand, a dichroic filter is also well-known which has a dielectric thin film comprised of alternately laminated H layers each formed of a dielectric thin film with high refractive index and L layers each formed of a dielectric thin film with low refractive index.
The dichroic film also has the optical characteristic of utilizing interference in the multilayer film through reflection at the interface between the H layer and the L layer to transmit or reflect light in a specific wavelength band. A variety of film structures are known which realize the functions of a high pass filter, a low pass filter, a band pass filter or the like. In particular, to separate the wavelength bands for red, green, and blue from each other, it is possible to use a long wavelength transmission filter, a wavelength band pass filter, a short wavelength transmission filter or the like. The characteristic of the dichroic filter is degraded when the incident angle and polarization conditions are out of design conditions.
When the incident angle is changed, the optical admittance of the thin film material is changed to widen the transmission band of p-polarized light (or narrow the reflection band) and narrow the transmission band of s-polarized light (or widen the reflection band). As a result, the transition wavelengths at the shift from the transmission band to the reflection band are changed in opposite directions in the p-polarized light and s-polarized light.
Thus, design is typically made such that the number of the layers in the polarization splitting film is added or the thickness of the film is modified to widen the angular range for use and reduce the polarization dependence in p-polarized light and s-polarized light. On the contrary, design may be made by taking advantage of the difference in characteristics depending on polarization.
The PBS or dichroic filter is used to form a color separation/combination optical system (a color separation/color combination means) of an image projection apparatus.
FIG. 27 shows an example of a conventional image projection apparatus which employs a light modulator of a reflection type realized with liquid crystal.
Arrows represent the optical paths of light beams for red, green, and blue in white display (image information is for white color). Solid lines represent s-polarized light, while broken lines represent p-polarized light.
White light emits from a light source 51, and unified into s-polarized light by a polarization changer 52. A dichroic mirror 53a transmits a light beam 30 in a green wavelength band, and reflects a light beam 40 in a red wavelength band and a light beam 20 in a blue wavelength band.
The light beam 30 in the green wavelength band transmitted through the dichroic mirror 53a is reflected by a PBS 54a, incident on a reflection type light modulator 55g realized with liquid crystal for green, and modulated. For the white display, the modulated light emerges therefrom as p-polarized light 31 which is then transmitted through the PBS 54a and a PBS 54c and is incident on a projection lens system (a projection optical system) 57 for projection.
The light beam 20 in the blue wavelength band reflected by the dichroic mirror 53a is changed into p-polarized light 21 by a wavelength selective phase shifter 56b, transmitted through a PBS 54b, and incident on a reflection type light modulator 55b realized with liquid crystal for blue and then modulated.
For the white display, the modulated light emerges therefrom as s-polarized light 20, so that it is reflected by the PBS 54b and maintained as the s-polarized light 20 through a wavelength selective phase shifter 56r. It is then reflected by the PBS 54c and is incident on the projection lens system 57 for projection.
The light beam 40 in the red wavelength band reflected by the dichroic mirror 53a is maintained as the s-polarized light 40 through the wavelength selective phase shifter 56b, reflected by the PBS 54b, and incident on a reflection type light modulator 55r realized with liquid crystal for red, and then modulated. For the white display, since the modulated light emerges therefrom as p-polarized light 41, it is transmitted through the PBS 54b, changed into s-polarized light 40 through the wavelength selective phase shifter 56r, reflected by the PBS 54c, and incident on the projection lens system 57 for projection.
For black display (image information is for black color), all of the light beams emerge from the reflection type light modulators 55r, 55g, or 55b with the same polarization as when they are incident thereon, so that they return toward the light source 51 along the same optical paths through the respective optical members. The color separation/combination means as described above is used to take advantage of the reflection type light modulator realized with the liquid crystal with high resolution and to form a small apparatus.
The conventional PBS aims to transmit p-polarized light and reflect s-polarized light throughout the wavelength band for use. The PBS described in Document 1 reflects p-polarized light and transmits s-polarized light. All of the PBSs are devices for providing polarization splitting throughout the wavelength band for use. None of the previously reported PBSs has wavelength selectivity or has polarization splitting characteristics reversed in different wavelength bands such that it transmits s-polarized light and reflects p-polarized light in a first wavelength band and reflects s-polarized light and transmits p-polarized light in a second wavelength band different from the first wavelength band.
In the dichroic filter, the transmission band of p-polarized light is widened and the transmission band of s-polarized light is narrowed when light is obliquely incident thereon, so that polarization splitting is performed in a certain wavelength band. However, p-polarized light is transmitted and s-polarized light is reflected at all times, and p-polarized light is not reflected and s-polarized light is not transmitted. Thus, it does not have the characteristic depending on wavelength such that it transmits s-polarized light and reflects p-polarized light in a first wavelength band and reflects s-polarized light and transmits p-polarized light in a second wavelength band different from the first wavelength band.
The color separation/combination in the image projection apparatus shown in FIG. 27 requires the two wavelength selective phase shifters 56b and 56r. Each of the wavelength selective phase shifters 56b and 56r is comprised of a plurality of laminated stretched polycarbonate films with birefringence such that their anisotropy axes are arranged at particular angles, as described in U.S. Pat. No. 5,658,490 (hereinafter referred to as “Document 2”). It involves a more complicated fabricating method and thus is an expensive optical device as compared with the PBS or dichroic filter formed with the dielectric thin film using a deposition method.
Since the polycarbonate is a polymer film, it is highly susceptible to the influence of external environment such as heat, humidity, and ultraviolet rays in view of the physical property of the material, and the reliability and durability of the color separation/combination means may be reduced. In addition, the low surface accuracy may cause flare if it is used in an optical system. On the other hand, depending on an anti-reflection film, each of the projection lenses reflects some amount of light, and the reflected light returns toward the color separation/combination means as return light. In the conventional color separation/combination means, the return light reaches the light modulator realized with liquid crystal and thus causes flare.
Japanese Patent Laid-Open No. 11-504441 (hereinafter referred to as “Document 3”) has disclosed a projection apparatus which employs a reflection type light modulator realized with liquid crystal without a wavelength selective phase shifter. Document 3 has disclosed a color separation/combination means which employs a PBS having the effect of reflecting p-polarized light and transmitting s-polarized light in a blue wavelength band and transmitting p-polarized light and reflecting s-polarized light in green and red wavelength bands. However, the PBS is only described in terms of its functions, and a method of realizing it has not been disclosed.
A polarization beam splitter with wavelength selectivity (a wavelength selective polarization beam splitter) which transmits s-polarized light and reflects p-polarized light in a first wavelength band and reflects s-polarized light and transmits p-polarized light in a second wavelength band different from the first wavelength band has not been realized.
It is an object of the present invention to provide a wavelength selective polarization beam splitter which transmits s-polarized light and reflects p-polarized light in a first wavelength band and reflects s-polarized light and transmits p-polarized light in a second wavelength band different from the first wavelength band.
It is another object of the present invention to realize a projection apparatus which employs the wavelength selective polarization beam splitter as a color separation/color combination means (a color separation/combination system) of the projection apparatus to simplify the structure and achieve high contrast with excellent reliability and durability.